Bowel cleaning fresh spatters , specks that stick to the sides and back of the bowl do not flush off as no water flows to these spots. Not enough water not good.

Mechanism that releases and shuts off water should be checked if any part has disconnect or improper connection.

Try taking the float bowl off and clean out the orfus. It will eather be in the nut holding the bowl on or in the end (sides) of the pickup tub the nut scews on.

Try lowering the float level. This will reduce the amount of water in the cistern and lower the pressure on the flapper so when you flush the water will not flow as fast into the bowl.

All toilet bowls have a preset water level determined buy the design of the internal s-trap and can not be change. However if the water level in the tank is not as high as it should be this will affect the water put into the bowel as the bowel in refilled by the flush valve and if it is set to stop before it get filled to the proper setting the bowl water level may be low.
A quick test is to see if the water level in the tank is up to the line in the tank most all toilet tanks have a line either stenciled or marked with a indentation in the porcelain. Another way is to mark the level in the bowel with tape or a marker then pour water slowly into the bowel not fast enough to flush it wait to see if it stays higher then the mark you made or settles back to that level.

Check to be sure small tube is flowing inside the ovefow tube. This is the water that flushes the rim holes, and refills the bowl. If you don't have flow clean or replace the float valve.

The only adjustment available is to control how full the tank will be for the next flush. The water that runs down into the bowl after flushing comes through the same valve that fills the tank, so the problem (and solution) is entirely with the amount of water that runs into the bowl while the tank is filling. The tank is filling sooner than enough replacement water can run into the bowl. Make sure that the tube (usually a 1/2 inch rubber tube) that runs from the float valve to the pipe that fills the bowl (usually a 1 inch pipe with an open top) is actually delivering all of it's flow down that pipe. If the flow of water being delivered by that tube is slow, you will have to find and eliminate the restriction. You might be able to pull of the rubbber tube off of the float valve and stick a small drill bit (just using your fingers, not a drill) into the fitting where you pulled off the tube.

The fill valve needs replacement unless you can take the top off snd find out what is keeping it from closing. I had one I took apart for the same reason and found a piece of plastic in the valve seat. Good Luck, Ned

make sure water level is sat at factory settings. there should be a black line that reads WATER LEVEL. If this is not the problem. then you have calci buildup. get a gal. of muratic acid,plunge all water out of toilet bowl. then dump in at least 1/2 gal. let set over nite. with water turned off. get up turn on water then flush. You mite have to do it twrice. hope this helps you.... duane.

Take the strainer at the end of the hose on the tank off. Throughly clean it. I used an old tooth brush, then ran hot water through it several times. Also drained the tank and flushed it out 3-4 times with fresh water, then ran fresh water through the pump for several minutes

Hope this helps

Make a small mark with a pencil or marker that can be erased of were your water line is in your bowl. Pour some water from a bucket into the bowl SLOWLY and see if the water line rises or drops back down to your original mark. If it drop back to the original mark, then that is the way the toilet was made. You won't be able to do anything about it. If it rises and stays at a higher level, then you have a problem with the fill tube in the tank. You will need to call a Pro!